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Fiber Manufacturing Technology

Synthetic and Cellulosic Fiber Formation Technology

Most synthetic and cellulosic manufactured fibers are created
by extrusion  forcing a thick, viscous liquid
(about the consistency of cold honey) through the tiny holes of a
device called a spinneret to form continuous filaments of
semi-solid polymer.

In their initial state, the fiber-forming polymers are solids
and therefore must be first converted into a fluid state for
extrusion. This is usually achieved by melting, if the polymers
are thermoplastic synthetics (i.e., they soften and melt when
heated), or by dissolving them in a suitable solvent if they
are non-thermoplastic cellulosics. If they cannot be dissolved
or melted directly, they must be chemically treated to form soluble
or thermoplastic derivatives. Recent technologies have been developed
for some specialty fibers made of polymers that do not melt,
dissolve, or form appropriate derivatives. For these materials,
the small fluid molecules are mixed and reacted to form the otherwise
intractable polymers during the extrusion process.

The Spinneret
The spinnerets used in the production of most manufactured
fibers are similar, in principle, to a bathroom shower head.
A spinneret may have from one to several hundred holes. The tiny
openings are very sensitive to impurities and corrosion. The
liquid feeding them must be carefully filtered (not an easy task
with very viscous materials) and, in some cases, the spinneret
must be made from very expensive, corrosion-resistant metals.
Maintenance is also critical, and spinnerets must be removed
and cleaned on a regular basis to prevent clogging.

As the filaments emerge from the holes in the spinneret, the
liquid polymer is converted first to a rubbery state and then
solidified. This process of extrusion and solidification of endless
filaments is called spinning, not to be confused with the textile
operation of the same name, where short pieces of staple fiber
are twisted into yarn. There are four methods of spinning filaments
of manufactured fibers: wet, dry, melt, and gel spinning.

Wet Spinning
Wet spinning is the oldest process. It is used for
fiber-forming substances that have been dissolved in a solvent.
The spinnerets are submerged in a chemical bath and as the filaments
emerge they precipitate from solution and solidify.

Because the solution is extruded directly into the precipitating
liquid, this process for making fibers is called wet spinning.
Acrylic,
rayon,
aramid,
modacrylic
and spandex
can be produced by this process.

Dry Spinning
Dry spinning is also used for fiber-forming substances
in solution. However, instead of precipitating the polymer by
dilution or chemical reaction, solidification is achieved by
evaporating the solvent in a stream of air or inert gas.

Melt Spinning
In melt spinning, the fiber-forming substance is melted for extrusion
through the spinneret and then directly solidified by cooling.
Nylon,
olefin,
polyester,
saran
and sulfar
are produced in this manner.

Melt spun fibers can be extruded from the spinneret in different
cross-sectional shapes (round, trilobal, pentagonal, octagonal,
and others). Trilobal-shaped fibers reflect more light and give
an attractive sparkle to textiles.

Gel Spinning
Gel spinning is a special process used to obtain high
strength or other special fiber properties. The polymer is not
in a true liquid state during extrusion. Not completely separated,
as they would be in a true solution, the polymer chains are bound
together at various points in liquid crystal form. This produces
strong inter-chain forces in the resulting filaments that can
significantly increase the tensile strength of the fibers. In
addition, the liquid crystals are aligned along the fiber axis
by the shear forces during extrusion. The filaments emerge with
an unusually high degree of orientation relative to each other,
further enhancing strength. The process can also be described
as dry-wet spinning, since the filaments first pass through air
and then are cooled further in a liquid bath. Some high-strength
polyethylene and aramid fibers are produced by gel spinning.

Stretching and Orientation
While extruded fibers are solidifying, or in some
cases even after they have hardened, the filaments may be drawn
to impart strength. Drawing pulls the molecular chains together
and orients them along the fiber axis, creating a considerably
stronger yarn.